Supercapacitors are electrochemical energy-storage devices that store charge by reversible adsorption of ions onto high-surface area, porous materials (known as “electric double layer capacitors”) or reversible surface reduction-oxidation (redox) reactions (known as “pseudo-capacitors”). With their high power density and long cycle stability, supercapacitors are well-suited to complement or replace batteries in a wide range of applications, including transportation, renewable energy, and portable electronics. High-performance supercapacitors are characterized by high specific capacitance, good stability over repeated cycling, and low series resistance.